There should be a "knob" on the back of the marker (lower tube) if you screw it in, it should increase the velocity. There may be a small set screw on the knob that you may have to back out to screw it in.

The set screw should be on the knob itself, if it's still there. If the knob is turned in all the way and you're still not getting a high enough velocity to play safely then you could try replacing the main spring. Try a Maddmann spring kit (if Maddmann are still around).